Mass spectrometer and methods of mass spectrometry

ABSTRACT

A way of increasing the dynamic range of a mass spectrometer which incorporates a time to digital converter such as commonly used with a time of flight mass analyser is disclosed. A z-lens upstream of the analyser can be switched between a high sensitivity mode wherein a beam of ions passing therethrough is substantially focused on to the entrance slit of the analyser, and a low sensitivity mode wherein the beam of ions is defocused so that the diameter of the beam substantially exceeds that of the entrance slit of the analyser. Obtaining data in the low sensitivity mode in combination with obtaining data in the high sensitivity mode enable an order of magnitude increase in the dynamic range to be obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to mass spectrometers and methodsof mass spectrometry.

[0003] 2. Discussion of the Prior Art

[0004] Various types of mass spectrometers are known which use a massanalyser which incorporates a time to digital converter (“TDC”) alsoknown as an ion arrival counter. Time to digital converters are used,for example, in time of flight mass analysers wherein packets of ionsare ejected into a field-free drift region with essentially the samekinetic energy. In the drift region, ions with different mass-to-chargeratios in each packet of ions travel with different velocities andtherefore arrive at an ion detector disposed at the exit of the driftregion at different times. Measurement of the ion transit-time thereforedetermines the mass-to-charge ratio of that particular ion.

[0005] Currently, one of the most commonly employed ion detectors intime of flight mass spectrometers is a single ion counting detector inwhich an ion impacting a detecting surface produces a pulse of electronsby means of, for example, an electron multiplier. The pulse of electronsis typically amplified by an amplifier and a resultant electrical signalis produced. The electrical signal produced by the amplifier is used todetermine the transit time of the ion which struck the detector by meansof a time to digital converter which is started once a packet of ions isfirst accelerated into the drift region. The ion detector and associatedcircuitry is therefore able to detect a single ion impacting onto thedetector.

[0006] However, such ion detectors exhibit a certain dead-time followingan ion impact during which time the detector cannot respond to anotherion impact. A typical detector dead time may be of the order of 1-5 ns.If during acquisition of a mass spectrum ions arrive during the detectordead-time then they will consequently fail to be detected, and this willhave a distorting effect on the resultant mass spectra.

[0007] It is known to use dead time correction software to correct fordistortions in mass spectra. However, software correction techniques areonly able to provide a limited degree of correction. Even after theapplication of dead time correction software, ion signals resulting inmore than one ion arrival on average per pushout event at a given massto charge value will result in saturation of the ion detector and henceresult in a non-linear response and inaccurate mass determination.

[0008] This problem is particularly accentuated with gas chromatographyand similar mass spectrometry applications because of the narrowchromatographic peaks which are typically presented to the massspectrometer which may be, for example, 2 seconds wide at the base.

[0009] Known time of flight mass spectrometers therefore suffer from alimited dynamic range especially in certain particular applications.

[0010] It is therefore desired to provide an improved mass spectrometerand methods of mass spectrometry.

[0011] The mass spectrometer according to the preferred embodimentenables the dynamic range of the detector to be extended. In particular,it is possible to alternate between two or more sensitivity rangesduring an acquisition. One range is tuned to have a high sensitivity. Asecond range is adjusted to be at a lower sensitivity than the firstrange by a factor of up to ×100. Preferably, the difference insensitivity between the first and second sensitivity modes is at least afactor ×10, ×20, ×30, ×40, ×50, ×60, ×70, ×80, ×90 or ×100.

[0012] Exact mass measurements can be made using a single point lockmass common to both high and low sensitivity ranges.

[0013] Although in the preferred embodiment the sensitivity is changedby the operation of a z-lens, other embodiments are also contemplatedwherein in a more general arrangement, the ion optical system betweenthe ion source and the mass analyser is altered or changed so that ionspassing therethrough are focused/defocused thereby altering the iontransmission efficiency. It is possible to change the ion transmissionefficiency by a number of methods, including: (i) altering a y-focusinglens, which may be an Einzel lens; (ii) altering a z-focusing lens,which may be an Einzel lens; (iii) using a stigmatic focusing lens,preferably having a circular aperture, which focuses/defocuses an ionbeam in both the y- and z-directions; and (iv) using a dc quadrupolelens which can focus/defocus in the y-direction and/or the z-directionas desired.

[0014] Utilising z-focusing is preferred to other ways of altering theion transmission efficiency since it has been found to minimise anychange in resolution, mass position and spectral skew which otherwiseseem to be associated with focusing/deflecting the ion beam in they-direction. However, in less preferred embodiments the ion beam may bealtered in the y-direction either instead of the z-direction or inaddition to the z-direction.

[0015] At least an order of magnitude increase in the dynamic range canbe achieved with the preferred embodiment. It has been demonstrated thatthe dynamic range can be extended from about 3.25 orders of magnitude toabout 4.25 orders of magnitude with a GC (gas chromatography) peak widthof about 1.5 s at half height.

[0016] Preferably, the ion source is a continuous ion source. Furtherpreferably, the ion source is selected from the group comprising: (i) anelectron impact (“EI”) ion source; (ii) a chemical ionisation (“CI”) ionsource; and (iii) a field ionisation (“FI”) ion source. All these ionsources may be coupled to a gas chromatography (GC) source.Alternatively, and particularly when using a liquid chromatography (LC)source either an electrospray or an atmospheric pressure chemicalionisation (“APCI”) ion source may be used.

[0017] Preferably, the mass analyser comprises a time to digitalconverter.

[0018] Preferably, the mass analyser is selected from the groupcomprising: (i) a quadrupole mass analyser; (ii) a magnetic sector massanalyser; (iii) an ion trap mass analyser; and (iv) a time of flightmass analyser, preferably an orthogonal acceleration time of flight massanalyser.

[0019] Preferably, the mass spectrometer further comprises control meansarranged to alternately or otherwise regularly switch the z-lens, ormore generally the ion optics, back and forth between at least first andsecond modes. In this embodiment, two data streams are stored as twodiscrete functions presenting two discrete data sets. Once the ratio ofthe high sensitivity to low sensitivity data has been determined, thedata can be used to yield linear quantitative calibration curves overfour orders of magnitude. Furthermore, the system can be arranged sothat exact mass data can be extracted from either trace. Therefore, if aparticular eluent produces a mass spectral peak which is saturated inthe high sensitivity data set and therefore exhibits poor massmeasurement accuracy, the same mass spectral peak may be unsaturated andcorrectly mass measured in the lower sensitivity trace. By using acombination of both traces, as a sample elutes exact mass measurementsmay be produced over a wide range of sample concentration.

[0020] The relative dwell times in the high and low sensitivity modesmay either be the same, or in one embodiment more time may be spent inthe higher sensitivity mode than in the lower sensitivity mode. Forexample, the relative time spent in a high sensitivity mode comparedwith a low sensitivity mode may be at least 50:50, 60:40, 70:30, 80:20,or 90:10. In other words, at least 50%, 60%, 70%, 80% or 90% of the timemay be spent in the higher sensitivity mode compared with the lowersensitivity mode.

[0021] Alternatively, the control means may be arranged to switch thez-lens, or more generally the ion optics, from the first mode to thesecond mode when the detector is approaching or experiencing saturationand/or to switch the z-lens, or more generally the ion optics, from thesecond mode to the first mode when a higher sensitivity is possiblewithout the detector substantially saturating in the first mode.According to the preferred embodiment, low mass peaks may be ignored inthe determination of whether or not to switch sensitivities and in oneembodiment it is only if mass peaks falling within a specific mass tocharge range (e.g. m/z 50, or 75, or 100) saturate or approachsaturation that the control means switches sensitivity modes.Additionally/alternatively to ignoring saturation of low mass peaks andconcentrating on mass peaks in one or more specific mass ranges (whichare preferably predefined, but in less preferred embodiments do notnecessarily need to be), the control means may switch sensitivity modesbased upon whether specific, preferably predetermined, mass peaks areapproaching saturation or are saturated, or if an improved mass spectrumincluding that specific mass peak could be obtained by switching to adifferent sensitivity mode.

[0022] Preferably, the mass spectrometer further comprises a powersupply capable of supplying from −100 to +100V dc to the z-lens. In oneembodiment, the z-lens may be a three part Einzel lens wherein the frontand rear electrodes are maintained at substantially the same dc voltage,e.g. for positive ions around −40V dc, and an intermediate electrode maybe varied, for positive ions, from approximately −100V dc in the highsensitivity (focusing) mode anywhere up to approximately +100V dc in thelow sensitivity (defocusing) mode. For example, in the low sensitivitymode a voltage of −50V dc, +0V dc, +25V dc, +50V dc or +100V dc may beapplied to the central electrode.

[0023] Preferably, when the z-lens defocuses a beam of ions passingthrough the z-lens, the beam of ions is diverged to have a profile orarea which substantially exceeds the profile or area of an entranceaperture to the mass analyser by at least a factor ×2, ×4, ×10, ×25,×50, ×75, or ×100.

[0024] Preferably, in the first mode at least 85%, 90%, 95%, 96%, 97%,98%, 99% or substantially 100% of the ions are arranged to pass throughthe entrance aperture.

[0025] Preferably, in the second mode less than or equal to 15%, 10%,5%, 4%, 3%, 2%, or 1% of the ions are arranged to pass through theentrance aperture.

[0026] According to one embodiment, the ion optical system is arrangedand adapted to be operated in at least three different sensitivitymodes. In yet further embodiments four, five, six etc. up to practicallyan indefinite number of sensitivity modes may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Various embodiments of the present invention will now bedescribed, by way of example only, and with reference to theaccompanying drawings in which:

[0028]FIG. 1 shows an arrangement of y-focusing lenses and a z-lensupstream of a mass analyser;

[0029]FIG. 2(a) shows a side view of a mass spectrometer according to apreferred embodiment;

[0030]FIG. 2(b) shows an additional side view of the mass spectrometerof FIG. 2(a);

[0031]FIG. 3 shows a plan view of a mass spectrometer coupled to a gaschromatograph; and

[0032]FIG. 4 shows experimental data illustrating the extended dynamicrange which is achievable with the preferred embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0033] A preferred embodiment of the present invention will now bedescribed. FIG. 1 shows an ion source 1, preferably an electron impactor chemical ionisation ion source. An ion beam 2 emitted from the ionsource 1 travels along an axis commonly referred to as the x-axis. Theions in the beam 2 are focused in a first y-direction as shown in theFigure by y-focusing and collimating lenses 3. A z-lens 4, preferablydownstream of the y-lens 3, is arranged to deflect or focus the ions ina second z-direction which is perpendicular to both the firsty-direction and to the x-axis. The z-lens 4 may comprise a number ofelectrodes, and may in one embodiment comprise an Einzel lens whereinthe front and rear electrodes are maintained at substantially the samefixed dc voltage, and the dc voltage applied to an intermediateelectrode may be varied to alter the degree of focusing/defocusing of anion beam 2 passing therethrough. An Einzel lens may also be used for they-lens 3. In less preferred arrangements, either a z-lens 4 or a y-lens3 (but not both) may be provided.

[0034] FIGS. 2(a) and (b) show side views of a mass spectrometer. InFIG. 2(a) the beam of ions 2 emitted from an ion source 1 is shownpassing through the y-focusing and collimating lens 3. The z-lens 4operating in a first (higher sensitivity) mode focuses the beam 2substantially within the acceptance area and acceptance angle of anentrance slit 10 of the mass analyser 9 so that a substantial proportionof the ions (i.e. normal intensity) subsequently enter the analyser 9which is positioned downstream of the entrance slit 10.

[0035]FIG. 2(b) shows the z-lens 4 operating in a second (lowersensitivity) mode wherein the z-lens 4 defocuses the beam of ions 2 sothat the beam of ions 2 has a much larger diameter or area than that ofthe entrance slit 10 to the mass analyser 9. Accordingly, a much smallerproportion of the ions (i.e. reduced intensity) will subsequently enterthe analyser 9 in this mode of operation compared with the mode ofoperation shown in FIG. 2(a) since a large percentage of the ions willfall outside of the acceptance area and acceptance angle of the entranceslit 10.

[0036]FIG. 3 shows a plan view of a preferred embodiment. A removableion source 1 is shown together with a gas chromatography interface orreentrant tube 7 which communicates with a gas chromatography oven 6. Alock mass inlet is typically present but is not shown. A beam of ions 2emitted by the ion source 1 passes through lens stack and collimatingplates 3,4 which includes a switchable z-lens 4. The z-focusing lens 4is arranged in a field free region of the optics and is connected to afast switching power supply capable of supplying from −100 to +100V DC.With positive ions, −100 V dc will focus an ion beam 2 passingtherethrough and a more positive voltage, e.g. up to +100V dc, willsubstantially defocus a beam of ions 2 passing therethrough and therebyreduce the intensity of the ions entering the analyser 9.

[0037] Initially, the system may be tuned to full (high) sensitivity.The z-focusing lens voltage may then be varied, preferably manually,until the desired lower sensitivity is reached. In one embodiment,acquisition then results in fast switching of the z-lens power supplybetween two (or more) predetermined voltages so as to repetitivelyswitch between high and low sensitivity modes of operation. High and lowsensitivity spectra may be stored as separate flnctions to be postprocessed. In an alternative embodiment, the z-lens 4 only switchesbetween higher and lower sensitivity modes (and vice versa) when eitherthe detector 13 is being saturated in one mode or the sensitivity can beimproved in another mode without saturation.

[0038] Downstream of ion optics 3,4 is an automatic pneumatic isolationvalve 8. The beam of ions 2 having passed through ion optics 3,4 thenpasses through an entrance slit or aperture 10 into the analyser 9.Packets of ions are then injected into the drift region of thepreferably orthogonal acceleration time of flight mass analyser 9 bypusher plate 11. Packets of ions are then preferably reflected byreflectron 12. The ions contained in a packet are temporally separatedin the drift region and are then detected by detector 13 whichpreferably incorporates a time to digital converter in its associatedcircuitry.

[0039]FIG. 4 shows experimental data illustrating that the dynamic rangecan be extended from about 3.25 orders of magnitude to about 4.25 ordersof magnitude (for a GC peak width of 1.5s at half height) using acombination of data from both the high and low sensitivity data sets. Inthis particular case, the system was tuned to give a ratio ofapproximately 80:1 between the high and low sensitivity data sets. Theexperiment allowed equal acquisition time for both data sets byalternating between the two sensitivity ranges between spectra.

[0040] Standard solutions ranging in concentration from 10 pg to 100 ngof HCB (Hexachlorobenzene) were injected via the gas chromatograph. Thepeak area response (equivalent to the ion count) for the reconstructedion chromatogram of mass to charge ratio 283.8102 was plotted againstthe concentration. The results from the low sensitivity data set weremultiplied by ×80 before plotting to normalise them to the highsensitivity data set.

[0041] Although described with respect to a preferred embodiment of theinvention, it should be readily understood that various changes and/ormodifications can be made to the invention without departing from thespirit thereof. Instead, the invention is only intended to be limited bythe scope of the following claims.

1. A mass spectrometer comprising: an ion source (1) for emitting a beamof ions (2); collimating and/or focusing means (3) downstream of saidion source (1) for collimating and/or focusing said beam of ions (2) ina first (y) direction; a lens (4) downstream of said collimating and/orfocusing means (3) for deflecting and/or focusing said beam of ions (2)in a second (z) direction perpendicular to said first (y) direction; amass analyser (9) downstream of said lens (4), said mass analyser (9)having an entrance region (10) for receiving ions, said ions beingsubsequently transmitted through said mass analyser (9), said massanalyser (9) further comprising a detector (13); characterised in that:said lens (4) is arranged and adapted to be operated in at least a firstrelatively higher sensitivity mode and to automatically switch to asecond relatively lower sensitivity mode, wherein in said second modeions are defocused by said lens (4) so that substantially fewer ions arereceived in said entrance region (10) of said mass analyser (9) than insaid first mode.
 2. A mass spectrometer as claim in claim 1, whereinsaid ion source (1) is a continuous ion source.
 3. A mass spectrometeras claimed in claim 2, wherein said ion source (1) is selected from thegroup comprising: (i) an electron impact (“EI”) ion source; (ii) achemical ionisation (“CI”) ion source; and (iii) a field ionisation(“FI”) ion source.
 4. A mass spectrometer as claimed in claim 3, whereinsaid ion source (1) is coupled to a gas chromatograph.
 5. A massspectrometer as claimed in claim 2, wherein said ion source (1) isselected from the group comprising: (i) an electrospray ion source; and(ii) an atmospheric pressure chemical ionisation (“APCI”) source.
 6. Amass spectrometer as claimed in claim 5, wherein said ion source (1) iscoupled to a liquid chromatograph.
 7. A mass spectrometer as claimed inclaim 1, wherein said mass analyser (9) comprises a time to digitalconverter.
 8. A mass spectrometer as claimed in claim 1, wherein saidmass analyser (9) is selected from the group comprising: (i) aquadrupole mass analyser; (ii) a magnetic sector mass analyser; (iii) anion trap mass analyser; and (iv) a time of flight mass analyser,preferably an orthogonal acceleration time of flight mass analyser.
 9. Amass spectrometer as claimed in claim 1, further comprising controlmeans arranged to alternately or otherwise regularly switch said lens(5) back and forth between said first and second modes.
 10. A massspectrometer as claimed in claim 9, wherein said mass spectrometerspends substantially the same amount of time in said first mode as insaid second mode.
 11. A mass spectrometer as claimed in claim 9, whereinsaid mass spectrometer spends substantially more time in said first modethan in said second mode.
 12. A mass spectrometer as claimed in claim 1,further comprising control means arranged to switch said lens (4) fromsaid first mode to said second mode when said detector (13) isapproaching or at the limit of its sensitivity and/or to switch saidlens (4) from said second mode to said first mode when a highersensitivity is possible without said detector (13) substantiallysaturating.
 13. A mass spectrometer as claimed in claim 12, wherein saidcontrol means decides whether or not to switch said lens (4) betweenfirst and second modes and vice versa by considering whether or notpredefined mass peaks or mass peaks within one or more predefined massranges are approaching saturation or are substantially saturated.
 14. Amass spectrometer as claimed in claim 13, wherein said predefined massrange(s) includes a range having a mass to charge ratio (“m/z”) selectedfrom the group comprising: (i) m/z≧40; (ii) m/z≧50; (iii) m/z≧60; (iv)m/z≧70; (v) m/z≧80; (vi) m/z≧90; (vii) m/z≧100; and (viii) m/z≧110. 15.A mass spectrometer as claimed in claim 1, further comprising a powersupply capable of supplying from −100 to +100V dc to said lens (4). 16.A mass spectrometer as claimed in claim 1, wherein said lens (4) is anEinzel lens comprising a front, intermediate and rear electrode, withsaid front and rear electrodes being maintained, in use, atsubstantially the same dc voltage and said intermediate electrode beingmaintained at a different voltage to said front and rear electrodes. 17.A mass spectrometer as claimed in claim 16, wherein said front and rearelectrodes are arranged to be maintained at between −30 to −50V dc forpositive ions, and said intermediate electrode is switchable from avoltage in said first mode of ≦−80V dc, preferably approximately −100Vdc, to a voltage≧+0V dc, preferably approximately +100 V dc.
 18. A massspectrometer comprising: an electron impact (“EI”) or chemicalionisation (“CI”) ion source (1); one or more y-focusing lenses (3)downstream of said ion source (1); a z-lens (4) downstream of said ionsource (1); a mass analyser (9) downstream of the at least oney-focusing lens (3) and said z-lens (4), said mass analyser (9)comprising a time to digital converter; characterised in that: said massspectrometer further comprises control means for automaticallycontrolling said z-lens (4), wherein said control means is arranged toselectively automatically defocus a beam of ions (2) passing through thez-lens (4).
 19. A mass spectrometer as claimed in claim 18, wherein whensaid z-lens (4) defocuses a beam of ions (2) passing through the z-lens(4), the beam of ions (2) is diverged to have a profile whichsubstantially exceeds an entrance aperture (10) to said mass analyser(9).
 20. A mass spectrometer comprising: a continuous ion source (1); amass analyser (9) having an entrance aperture (10); a z-lens (5)disposed between said ion source (1) and said mass analyser (9);characterised in that: said z-lens (5) is arranged and adapted to beoperated in: (i) a first mode so as to focus a beam of ions (2) passingtherethrough so that at least 80% of said ions will substantially passthrough said entrance aperture (10); and (ii) a second mode so as todefocus a beam of ions (2) passing therethrough so that 20% or less ofsaid ions will substantially pass through said entrance aperture (10).21. A mass spectrometer as claimed in claim 20, wherein in said firstmode at least 85%, 90%, 95%, 96%, 97%, 98%, 99% or substantially 100% ofsaid ions are arranged to pass through said entrance aperture (10). 22.A mass spectrometer as claimed in claim 20 or 21, wherein in said secondmode less than or equal to 15%, 10%, 5%, 4%, 3%, 2%, or 1% of said ionsare arranged to pass through said entrance aperture (10).
 23. A massspectrometer as claimed in claim 20, wherein the difference insensitivity between said first mode and second mode is at least ×10,×20, ×30, ×40, ×50, ×60, ×70, ×80, ×90 or ×100.
 24. A mass spectrometercomprising: an ion source (1); a lens (4) downstream of said ion source(1); a detector (13) downstream of said lens (4); characterised in that:said lens (4) focuses a beam of ions in a first mode of operation andsubstantially defocuses a beam of ions in a second mode of operation,wherein a control means is arranged to automatically switch said lens(4) between said first and second modes of operation.
 25. A method ofmass spectrometry comprising: providing an ion source (1) for emitting abeam of ions (2); providing collimating and/or focusing means (3)downstream of said ion source (1) for collimating and/or focusing saidbeam of ions (2) in a first (y) direction; providing a lens (4)downstream of said collimating and/or focusing means (3) for deflectingor focusing said beam of ions (2) in a second (z) directionperpendicular to said first direction (y); providing a mass analyser (9)downstream of said lens (4), said mass analyser (9) having an entranceregion (10) for receiving ions, said ions being subsequently transmittedthrough said mass analyser (9), said mass analyser further comprising adetector (13); characterised in that said method further comprises thestep of: arranging and adapting said lens (4) so as to be operable in atleast a first relatively higher sensitivity mode and to automaticallyswitch to a second relatively lower sensitivity mode, wherein in saidsecond mode ions are defocused by said lens (5) so that substantiallyfewer ions are received in said entrance region (10) of said massanalyser (9) than in said first mode.
 26. A mass spectrometercomprising: an ion source (1) for emitting a beam of ions (2) along anx-axis; a z-lens (4) for deflecting and/or (de)focusing said beam ofions; a mass analyser (9) having an entrance acceptance profile;characterised in that: said z-lens (4) is arranged to automaticallyswitch between two modes: (i) a higher sensitivity mode wherein saidz-lens (4) focuses said beam of ions (2) so that >60%, preferably>75%,of said ions fall within the entrance acceptance profile of said massanalyser (9); and (ii) a lower sensitivity mode wherein said z-lens (4)defocuses said beam of ions (2) so that<40%, preferably<25%, of saidions fall within the entrance acceptance profile of said mass analyser(9).
 27. A mass spectrometer, comprising: an ion source (1) for emittinga beam of ions (2); an ion optical system (3,4) downstream of said ionsource (1) for focusing a beam of ions (2) passing therethrough; a massanalyser (9) downstream of said ion optical system (3,4); characterisedin that: said ion optical system (3,4) is arranged to automaticallyswitch between at least a first relatively higher sensitivity mode andat least a second relatively lower sensitivity mode, wherein in saidsecond mode ions are defocused by said ion optical system (3,4) so thatfewer ions are transmitted to said mass analyser (9) than in said firstmode.
 28. A mass spectrometer as claimed in claim 27, wherein said ionoptical system (3,4) is arranged to automatically switch between threeor more different sensitivity modes.
 29. A mass spectrometer as claimedin claim 27, wherein the ion optical system (3,4) is arranged to spendapproximately the same amount of time in said first mode as it spends insaid second mode.
 30. A mass spectrometer as claimed in claim 27,wherein the ion optical system (3,4) is arranged to spend substantiallymore time in said first relatively higher sensitivity mode than in saidsecond relatively lower sensitivity mode.
 31. A mass spectrometercomprising: an ion source (1) for emitting a beam of ions substantiallyalong an x-axis; ion optical means (3,4) for focusing and defocusingsaid beam of ions in a y-direction perpendicular to said x-axis and/orin a z-direction perpendicular to said y-direction and said x-axis; amass analyser (9) downstream of said ion optical means (3,4), said massanalyser (9) comprising a detector (13); characterised in that: saidmass spectrometer further comprises automatic control means forautomatically switching said ion optical means either: (i) repetitivelyand regularly between a first higher sensitivity mode wherein said ionoptical means (3,4) focuses an ion beam (2) passing therethrough and asecond lower sensitivity mode wherein said ion optical means (3,4)defocuses an ion beam (2) passing therethrough; or (ii) from a highersensitivity mode wherein said ion optical means (3,4) focuses an ionbeam (2) passing therethrough to a lower sensitivity mode wherein saidion optical means (3,4) defocuses an ion beam (2) passing therethrough,said switching between sensitivity modes being made upon determiningthat particular mass peaks in a mass spectrum or equivalent thereof aresaturating or approaching saturation and/or mass peaks within aparticular mass range in a mass spectrum or equivalent thereof aresaturating or approaching saturation.
 32. A mass spectrometercomprising: an ion source (1) for emitting a beam of ions substantiallyalong an x-axis; ion optical means (3,4) for focusing and defocusingsaid beam of ions in a y-direction perpendicular to said x-axis and/orin a z-direction perpendicular to said y-direction and said x-axis; amass analyser (9) downstream of said ion optical means (3,4), said massanalyser comprising a detector (13); characterised in that: said massspectrometer is arranged to automatically adjust said ion optical means(3,4) so that either: (i) data at a relatively high sensitivity and at arelatively low sensitivity are obtained at substantially the same time;or (ii) only data at a first sensitivity is obtained until it isautomatically determined that data at an improved second sensitivity maybe obtained, at which point data at said second sensitivity is thenobtained.
 33. A mass spectrometer as claimed in claim 32, wherein saidmass analyser (9) comprises a time of flight mass analyzer.
 34. A massspectrometer comprising: ion optical means (3,4) for selectivelyfocusing/defocusing a beam of ions (2) passing therethrough, said ionoptical means (3,4) being positioned upstream of a mass analyzer (9);and automatic control means for automatically varying said ion opticalmeans (3,4) thereby altering the focusing/defocusing of a beam of ions(2) passing therethrough so as to alter the intensity of ionssubsequently entering said mass analyzer (9).